As soon as RIVER PHOENIX was found dead of a heroin OD outside a LA nightclub, VIPER ROOM owner JOHNNY DEPP fled town– book.

Amazingly, this coming Halloween marks the 20th anniversary of rising star River Phoenix’s tragic death of a heroin overdose outside LA’s famed hotspot The Viper .  Now, in a stunning new book Phoenix pal and one of the last people to see River alive recalls the star’s final moments Oct. 31, 1993.

In “Running with Monsters,” former Thelonious Monster frontman and “Celebrity Rehab” therapist Bob Forrest writes of the grim details

According to the book, Phoenix was a frustrated rock star despite his proto-vegan “save the world” public image, River had barricaded himself with Red Hot Chili Peppers’ guitarist John Frusciante, engaging on a massive hard drug binge.

“(River) stayed with John for the next few days, and probably didn’t get a minute of sleep,” Forrest writes in the book. “The drug routine stayed pretty consistent for all of us. First, smoke crack or shoot coke directly into a vein for that ninety-second, electric brain-bell jangle. Then shoot heroin to get a grip and come down enough to be able to carry on a conversation for a few minutes before you start the cycle again.”

The night he died, “River and John stumbled in” to the Viper Room, Forrest writes.

Also present were Phoenix’s brother, Joaquin, and sister Rain; his girlfriend Samantha Mathis; Red Hot Chili Pepper Flea, AND Johnny Depp, ostensible Viper Room owner and its public face.

 In the book, Forrest writes, as soon as Phoenix and Frusciante landed, massive quantities of cocaine was passed around.

 “River was obviously wasted,” Forrest writes. While watching Jonny Depp’s house band perform, Forrest recalled that someone tapped him on the shoulder. He turned ‘round only to see a ghostly pale Phoenix.

“Bob,” he said, “I don’t feel so good. I think I’m OD’ing.”

According to the book, Phoenix got hold of himself and then wandered back into teeming morass/. A few minutes later someone shouted over the wah-wah wall of noise to call 911.

Phoenix had been dragged outside club – by someone. He lay on the sidewalk in  the those of a massive seizure.

When the EMTs arrived, Forrest writes, Phoenix was  barely alive, and Flea jumped in the ambulance to accompany the stricken star to Cedars-Sinai Hospital. The books author, Forrest jumped in his car and raced to the hospital.

When he got there, he saw Samantha  Mathis in the hall as tears cascaded down her porcelain face.

He knew then that Phoenix didn’t make it.

Forrest called Depp, who asked unawares, “How’s your friend?”

“Dude,” Forrest said. “River’s dead.”

After a long pause, Depp reportedly said, “Oh my God,” and quickly hung up.

According to Forrest, the next morning, Depp, along with everyone else who knew what happened that eerie night, immediately left LA.